There's a very good discussion about the experience of depression going on.
Depression is a powerful thing. Some of the comments there do it considerable justice. It is bleak; it effects a person's view of the self and world.
There is a standardized definition of depression, called a "depressive episode". Involved in the definition are such indicators as sad mood, alterations of sleep or appetite, general energy level, decreased interests, feelings of guilt and regret, suicidal thinking, worsening concentration, and a slowed or agitated movement.
While these are all well and good as a way of communicating between professionals and others, the criteria does not do justice to its experience. In my years of work and training, I have yet to meet two people who have described the experience of depression in the same way.
It is refreshing to see people discuss things in a real experiential manner. It is especially important when trying to explain what something like depression feels like to someone who has never had the need to consider it.
